Nonaqueous electrolyte secondary cells such as lithium ion secondary cells are high-voltage and high-energy-density cells that have less self-discharge and a smaller memory effect and can be significantly lightened, and therefore are suitably used for compact and easy-to-carry electrical and electronic equipment such as laptop computers, mobile phones, smart phones, tablet PCs, and ultrabook (registered trademark) PCs. Moreover, such nonaqueous electrolyte secondary cells are now coming into practical use as electric power supplies for a variety of applications from vehicle power supplies for driving used in automobiles and the like to stationary large power supplies.
In order to improve the energy density of nonaqueous electrolyte secondary cells, improvement in the technique for producing electrodes is important. In the case of electrodes of lithium ion secondary cells, for example, when a carbonaceous material such as cokes and carbons is used as a negative electrode active material for production of a negative electrode, a carbonaceous material is commonly powdered and dispersed in a solvent together with a binder to prepare a negative electrode mixture. Then, the negative electrode mixture was applied to a negative electrode current collector and, after removal of the solvent by drying, rolled out to give an electrode. Carbonaceous materials simply occluding and releasing lithium ions are herein referred to as active materials. Similarly, in production of positive electrodes, for example, a lithium-containing oxide is used as a positive electrode active material. The positive electrode active material is commonly powdered and dispersed in a solvent together with a conductive agent and a binder to prepare a positive electrode mixture. The positive electrode mixture is applied to a positive electrode current collector and, after removal of the solvent by drying, rolled out to produce a positive electrode. Polyvinylidene fluoride has been conventionally often used as a binder for lithium ion secondary cells.
For example, the technique disclosed in Patent Literature 1 is as follows. A positive electrode mixture is prepared by mixing a positive electrode active material such as a lithium-containing oxide (e.g., LiCoO2), a conductive agent such as graphite, and polyvinylidene fluoride. The positive electrode mixture is dispersed in N-methylpyrrolidone to give a slurry. The slurry is applied to an aluminum foil positive electrode current collector. Separately, a negative electrode mixture is prepared by mixing a negative electrode active material such as a carbonaceous material with polyvinylidene fluoride. The negative electrode mixture is dispersed in N-methylpyrrolidone to give a slurry. The slurry is applied to a copper foil negative electrode current collector. They each are dried and compression-molded by a roller pressing machine to be formed into an electrode sheet.
A Polyvinylidene fluoride resin, however, has poor adhesiveness to base materials such as metals, and the adhesion strength thereof is desired to be improved. In addition, electrode sheets containing a polyvinylidene fluoride binder are less flexible to easily cause problems of cracking thereof and removal of the electrode mixture from the electrode sheets when folded 180° in production of square cells and when rolled to a small dimension in production of cylindrical cells, resulting in a poor yield. Moreover, a polyvinylidene fluoride resin problematically has poor alkali resistance to be easily gelled under alkaline conditions.
Introduction of functional groups such as carboxy to polyvinylidene fluoride resin has been considered for improvement of the adhesiveness of the polyvinylidene fluoride resin to base materials such as metals. For example, disclosed is a polar vinylidene fluoride copolymer prepared by copolymerizing a monomer mainly containing vinylidene fluoride and a small amount of unsaturated dibasic monoester or vinylenecarbonate (see Patent Literature 2). In Examples of Patent Literature 2, a copolymer of vinylidene fluoride and a monomer having carboxy or carbonate is mentioned.
Moreover, a binder prepared using a copolymer of vinylidene fluoride and tetrafluoroethylene has been also disclosed (see Patent Literature 3). Patent Literature 3 discloses that the use of such a copolymer improves the flexibility of a prepared binder. Patent Literature 3 also discloses that the binder prepared using a copolymer of vinylidene fluoride, tetrafluoroethylene, and a monomer having carboxy and carbonate has still better adhesiveness to a current collector.
According to another disclosure, in the case of containing a copolmyer of a specific acrylamide derivative and at least one monomer selected from vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, and hexafluoropropylene, such a paint or varnish is hard, stable, transparent in the absence of pigments and capable of forming a coating sufficiently adhering to metals or glass (see Patent Literature 4). The copolymer disclosed in Patent Literature 4 is used for a paint or varnish, and therefore is limited to those having a comparatively low solution viscosity.